Various well known medical techniques for the treatment of malignancies involve the use of radiation. Radiation sources, for example medical linear accelerators, are typically used to generate radiation which is directed to a specific target area of a patient. Proper doses of radiation directed at the malignant area of the patient are of the upmost importance. When properly applied, the radiation produces an ionizing effect on the malignant tissues of the patient, thereby destroying the malignant cells. As long as the dosimetry of the applied radiation is properly monitored, the malignancy can be treated without any detriment to the surrounding healthy body tissue. The goal of these treatments is to focus a high dose of radiation to a tumor or malignant cells while minimizing the exposure of the surrounding healthy tissue to the radiation. Accelerators may be utilized to deliver the radiation. Different accelerators have varying characteristics and output levels. The most common type of accelerator produces pulse radiation. The output beam has a rectangular shape in cross section and a cross sectional area typically between 1 and 1,600 square centimeters (cm2). Preferably, the cross sectional area or field size is between 1×1 square centimeters (cm2) and 40×40 square centimeters (cm2). Rectangular or square cross sectional shapes are often changed to any desired cross sectional shape using molded or cast lead or cerrobend materials. More advanced accelerators use multi-leaf collimators. Other accelerators are continuously or non-pulsed, such as cobalt radiation machines. Some accelerators utilize a swept electron beam, which passes a very narrow electron beam across the treatment field by means of varying electromagnetic fields.
To ensure proper dosimetry, linear accelerators used for the treatment of malignancies must be calibrated. Both the electron and photon radiation must be appropriately measured and correlated to the particular device. The skilled practitioner must insure that both the intensity and duration of the radiation treatment is carefully calculated and administered so as to produce the therapeutic result desired while maintaining the safety of the patient. Parameters such as flatness, symmetry, radiation and light field alignment are typically determined. The use of too much radiation may, in fact, cause side effects and allow destructive effects to occur to the surrounding tissue. Use of an insufficient amount of radiation will not deliver a dose that is effective to eradicate the malignancy. Thus, it is important to be able to determine the exact amount of radiation that will be produced by a particular machine and the manner in which that radiation will be distributed within the patient's body.
In order to produce an accurate assessment of the radiation received by the patient, at the target area, some type of pattern or map of the radiation at varying positions within the patient's body must be produced. These profiles correlate: 1) the variation of dose with depth in water generating percent depth dose profiles, 2) the variation of dose across a plane perpendicular to the radiation source generating the cross beam profiles, and 3) the variation of dose with depth in water generating percent depth dose and TMR/TPR (Tissue Maximum Ratio/Tissue Phantom Ratio) when the SAD (source to axis distance) is constant profiles. These particular measurements of cross beam profiles are of particular concern in the present invention. Although useful for other analyses, the alignment of the cross profiles in both radial and transverse planes are the basis of the present invention.
There are companies that provide the calibration service to hospitals and treatment centers. These physicists must visit the facility and conduct the calibration of the radiation source with their own equipment. This requires lightweight, easily portable, less cumbersome radiation measuring devices that can be quickly assembled and disassembled on site. The actual scanning should also be expeditious with the results available within a short time frame. Such equipment allows a physicist to be more efficient and calibrate more radiation devices in a shorter period of time.
One existing system for measuring the radiation that is produced by medical linear accelerators utilizes a large tank on the order of 50 cm×50 cm×50 cm filled with water. A group of computer controlled motors move the radiation detector through a series of pre-programmed steps along a single axis beneath the water's surface. Since the density of the human body closely approximates that of water, the water-filled tank provides an appropriate medium for creating a simulation of both the distribution and the intensity of radiation which would likely occur within the patient's body. The aforementioned tank is commonly referred to as a water phantom. The radiation produced by the linear accelerator will be directed into the water in the phantom tank, at which point the intensity of the radiation at varying depths and positions within the water can be measured with the radiation detector. As the radiation penetrates the water, the direct or primary beam is scattered by the water, in much the same way as a radiation beam impinging upon the human patient. Both the scattered radiation, as well as the primary radiation, are detected by the ion-chamber, which is part of the radiation detector or by radiation sensitive diodes.
The ion-chamber is essentially an open air capacitor which produces an electrical current that corresponds to the number of ions produced within its volume. The detector is lowered to a measurement point within the phantom tank and measurements are taken over a particular time period. The detector can then be moved to another measurement point where measurements are taken as the detector is held in the second position. At each measuring point a statistically significant number of samples are taken while the detector is held stationary.
In radiation therapy and radiosurgery, for example, a tumor may be non-invasively destroyed by a beam of ionizing radiation that kills the cells in the tumor. It is desirable to direct the radiation beam only to the tumor and not to the healthy tissue which surrounds the tumor. Therefore, accurate aiming of the beam at the tumor is extremely important in these radiation treatments. The goal is to focus a high dose of radiation to the tumor while minimizing the exposure of the surrounding healthy tissue to radiation. For adequate distribution of radiation dosage to the tumor, the direction of the radiation beam is typically adjusted during the treatment to track the tumor.
The most advanced modern radiosurgery systems, such as the Cyberknife® Robotic Radiosurgery System of Accuray, Inc., utilizes stereo online x-ray imaging during treatment to enhance the accuracy of the radiation treatment. The position of a patient's bony landmarks, e.g. their skull, can be determined with high accuracy by using the Cyberknife® stereo x-ray camera system. Thus, this highly accurate x-ray camera system can be used to treat a target region if the position of the target region relative to a bony landmark remains constant. However, the x-ray camera system cannot be used to determine the position of a target region if the position of the target region relative to a bony landmark changes because the target, e.g. a tumor, is generally not visible in x-ray images. For example, a target region in a patient's abdomen or chest cannot be treated with this method alone.
An image guidance system is essential to the proper operation of the Cyberknife® system. The first method developed for controlling the image guidance system was known as 6D or skull based tracking. An X-ray camera produces images which are compared to a library of computer generated images of the patient anatomy Digitally Reconstructed Radiographs (DRR's), and a computer algorithm determines what motion corrections have to be given to the robot because of patient movement. This imaging system allows the Cyberknife® to deliver radiation with an accuracy of 0.5 mm without using mechanical clamps attached to the patient's skull. The use of the image guided technique is referred to as frameless stereotactic radiosurgery. This method is referred to as 6D because corrections are made for the 3 translational motions (X, Y and Z) and three rotational motions.